Integrated circuit designers have always sought the ideal semiconductor memory: a device that is randomly accessible, can be written or read very quickly, is non-volatile, but indefinitely alterable, consumes little power, and is scalable. Emerging energy-absorption-related memories such as variable resistance memories increasingly offer these advantages. Programmable Conductance Random Access Memory (PCRAM) is one example of such a memory. Additionally, Magnetoresistive Random Access Memory (MRAM) technology has been viewed as offering all these advantages. Other types of variable resistance memories include polymer-based memory, chalcogenide-based memory, differential negative resistance (DNR) memory, and perskovite memory.
A PCRAM element has a structure including a chalcogenide-based glass region incorporating a metal (or metal ions) and electrodes on either side of the glass region. Information can be stored as a digital “1” or “0” as stable resistance states. A typical chalcogenide glass used in PCRAM devices is GexSe100-x. The chalcogenide glass can also be used in conjunction with layers of Ag and/or Ag2Se. An example of a PCRAM device is described in U.S. Pat. No. 6,348,365 to Moore and Gilton. The glass region of a PCRAM element can be made less resistive upon application of a threshold voltage. This less resistive state is maintained in a non- or semi-volatile manner and is reversible by applying a reversed voltage. The resistance state of a PCRAM element can be sensed by the application of a sub-threshold voltage through the cell element.
A magnetic memory element has a structure which includes ferromagnetic layers separated by a non-magnetic barrier layer that forms a tunnel junction. An example of an MRAM device is described in U.S. Pat. No. 6,358,756 to Sandhu et al. Information can be stored as a digital “1” or a “0” as directions of magnetization vectors in these ferromagnetic layers. Magnetic vectors in one ferromagnetic layer are magnetically fixed or pinned, while the magnetic vectors of the other ferromagnetic layer are not fixed so that the magnetization direction is free to switch between “parallel” and “antiparallel” states relative to the pinned layer. In response to parallel and antiparallel states, the magnetic memory element represents two different stable resistance states, which are read by the memory circuit as either a “1” or a “0.” Passing a current through the MRAM cell enables detection of the resistance states.
As mentioned above, polymer memory, another type of variable resistance memory, utilizes a polymer-based layer having ions dispersed therein or, alternatively, the ions may be in an adjacent layer. The polymer memory element is based on polar conductive polymer molecules. The polymer layer and ions are between two electrodes such that upon application of a voltage or electric field the ions migrate toward the negative electrode, thereby changing the resistivity of the memory cell. This altered resistivity can be sensed as a memory state.
Chalcogenide memory, another type of variable resistance memory, switches resitivity states by undergoing a phase change in response to resistive heating. The two phases corresponding to the two stable resistivity states include a polycrystalline state and an amorphous state. The amorphous state is a higher resistive state, which can be read as stored data.
DNR memory can be programmed to store information as an absolute DNR current maximum, thereby forming a memory element. The DNR memory element functions by storing data as separate, maintainable maximum current states, which are programmed when voltages are applied to the memory element. The observable DNR memory of such device is highly stable, repeatable, and predictable, making for an excellent memory device. An example of a DNR memory is described in U.S. patent application Ser. No. 10/410,567, filed Apr. 10, 2003, by Kristy A. Campbell.
The search for non-volatile memory devices has led to investigations into atomic-level properties of materials for switching and memory applications. Studies have been conducted into electron spin transistors and memory components. Even in the absence of a magnetic field, some materials exhibit splitting of the electron spin energy levels. This is referred to as zero field splitting. Zero field splitting is different from Zeeman splitting (i.e., separation of the electron spin energy levels in the presence of an externally applied magnetic field). The difference being that some molecules may exhibit splitting of the electron energy levels at zero externally applied magnetic field, due in part, to the natural crystal fields present around a metal ion (in the case of molecules with transition metal ions) or to spin-spin coupling within a molecule or between molecules. Molecules with transition metals (e.g., Mn, V, Fe, Co, Cr, Ni, Cu, Zn, Cd, and others) are quite frequently paramagnetic and may have electron spin energy levels at zero magnetic field with an energy splitting between levels for which a spin transition is allowed that is within a range detectable with microwave radiation. For example, as shown in FIG. 5, Mn+3 ions have a spin system with an effective spin S=2, with a positive zero field splitting value. The inset portion of FIG. 5 is an expanded view of the Ms=±2 energy levels in the region of observed parallel mode electron paramagnetic resonance transitions (indicated by the double arrows). Analytical techniques, such as microwave spectroscopy or electron paramagnetic resonance (EPR) spectroscopy can identify molecular systems that exhibit zero field splitting properties.
Spin-spin interactions occur when there is at least one unpaired electron interacting with another unpaired electron (S greater than or equal to 1, where S is the effective spin). An example molecular system that could give rise to this situation includes a molecule containing Mn+3, which has a total spin S=2 (e.g., the molecule Mn(salen)). In this case, there are 4 unpaired electrons interacting with each other.
Microwave absorption spectroscopy has been used to identify atomic properties of chemical species. Microwave absorption has been shown to be a viable means of determining energy absorption at frequencies corresponding to the zero field splitting value of the absorbing material.
Each of the above-discussed memory types utilize some energy absorbing property for storing information. Also, each can utilize a two terminal memory cell having a memory storing region separating two electrodes, which can be set up in a cross-point or modified cross-point style memory array, if desired.